This invention is an improvement in a process for the production of alkylated aromatic compounds.
Nearly forty years ago, it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured worldwide.
LABS are manufactured from linear alkyl benzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. The linear paraffins are straight chain (unbranched) or normal paraffins. Normally, the linear paraffins are a mixture of linear paraffins having different carbon numbers. The linear paraffins have generally from about 6 to about 22, preferably from 10 to 15, and more preferably from 10 to 12 or from 11 to 13, carbon atoms per molecule.
A preferred method of production of the linear paraffins is the extraction of straight chain hydrocarbons from a hydrotreated kerosene boiling range petroleum fraction. The kerosene boiling range fraction contains a mixture of different hydrocarbons, including mostly paraffinic and aromatic hydrocarbons, but containing also olefinic and naphthenic hydrocarbons. The kerosene boiling range fraction is usually defined as comprising a fraction having a boiling range of from about 300xc2x0 F. (149xc2x0 C.) to about 572xc2x0 F. (300xc2x0 C.). The initial boiling point of the kerosene boiling range fraction may vary from about 300 to about 374xc2x0 F. (149 to 190xc2x0 C.) and the final boiling point may vary from about 455 to about 572xc2x0 F. (235 to 300xc2x0 C.). The kerosene boiling range generally includes hydrocarbons having from about 8 to about 17 carbon atoms.
The kerosene boiling range fraction is generally produced by fractionating crude oil. Crude oil is the liquid part, after being freed from dissolved gas, of petroleum, a natural organic material composed principally of hydrocarbons that occur in geological traps. Being derived from a natural material, crude oils vary in composition depending on where the petroleum occurred and other factors. Commercial oil refineries typically receive crude oil from many different sources, and the composition of the crude oil that is charged to the crude oil fractionation unit changes frequently. The paraffinic and aromatic hydrocarbons that make up the bulk of a kerosene boiling range fraction can change as different crude oils are processed in the crude oil fractionation unit. It is common for the volumetric flow rate of the kerosene boiling range fraction to fluctuate by up to 30 vol-% or more, for a given boiling point range of the kerosene boiling range fraction produced from a crude oil fractionation unit. Because such fluctuations in flow rate can make it difficult to control downstream units that process the kerosene boiling range fraction, the operator of a crude oil fractionation unit may intentionally adjust the initial and final boiling points of the kerosene boiling range fraction as permissible within the above mentioned ranges and thereby achieve a more constant flow rate of the kerosene boiling range fraction. It is also common for the kerosene boiling range fraction to fluctuate between liquid phase and a mixture of vapor and liquid phases, since the proportion of vapor phase depends on both the composition of the kerosene boiling range fraction and its temperature, which can also vary. For a given temperature, the lighter the kerosene boiling range fraction, the greater is the proportion in the vapor phase. Accordingly, in commercial practice the composition, the amount, the boiling range and/or the phase of the kerosene boiling range fraction recovered from a commercial crude oil fractionation unit often fluctuate daily, or even hourly.
In order to produce LAB having, for example, from 11 to 13 carbon atoms per linear alkyl group, a stream of linear paraffins comprising C11 to C13 hydrocarbons is desired. A suitable stream is a heartcut of the kerosene boiling range fraction suffices, provided that hydrocarbons boiling lower than C11 linear paraffins and hydrocarbons boiling higher than C13 linear paraffins must be removed from the kerosene fraction. Generally, this heartcut is produced in a two-step, strip-and-rerun fractionation process. First, the kerosene fraction is introduced into a fractionation column, called a stripper column, which strips overhead the C10xe2x88x92 hydrocarbons from the kerosene feedstock, producing a bottom stream comprising C11+ hydrocarbons. Then, the bottom stream is introduced into a second fractionation column, called a rerun column, which boils overhead the C11 to C13 hydrocarbons as a heartcut and produces a bottom stream comprising C14+ hydrocarbons. In some commercial units, the overhead condenser of the second fractionation column is a contact condenser. This heartcut is then hydrotreated, and the straight chain hydrocarbons are extracted from the hydrotreated fraction, thereby producing the linear paraffin stream.
Alkylaromatic processes that use the two-step, strip-and-rerun fractionation process to produce the heartcut are inefficient, since they require relatively large amounts of utilities. Thus, alkylation processes are sought in which the heartcut is produced in a more efficient manner that uses fewer utilities than the prior art process.
Over fifty years ago, Wright proposed replacing two distillation columns with a single distillation column having a vertical partition (dividing wall column) within the column that would effect the separation of the column feed into three constituent fractions. It was recognized then that a dividing wall column could minimize the size or cost of the equipment needed to produce overhead, bottoms, and sidedraw products. See U.S. Pat. No. 2,471,134 (Wright). Wright described using the dividing wall column to separate a mixture of ethane, propane, butanes, and a small amount of C5 and heavier hydrocarbons.
Since then, researchers have studied the dividing wall column and have proposed using dividing wall columns for separating other mixtures, including xylenes (Int. Chem. Engg., Vol. 5, No. 3, July 1965, 555-561); butanes and butenes (See e.g., Trans IChemE, Vol.70, Part A, March 1992, 118-132); methanol, isopropanol, and butanol (See e.g., Trans IChemE, Vol. 72, Part A, September 1994, 639-644); ethanol, propanol, and butanol (Ind. Eng. Chem. Res. 1995, 34, 2094-2103); air (See e.g., Ind. Eng. Chem. Res. 1996, 35, pages 1059-1071); natural gas liquids (Chem. Engg., July 1997, 72-76); and benzene, toluene, and ortho-xylene (Paper No. 34 K, by M. Serra et al., prepared for presentation at the AlChE Meeting, Los Angeles, Calif., U.S.A., November 1997). The Serra et al. paper also describes separating mixtures of butanes and pentane; pentanes, hexane, and heptane; and propane and butanes.
Despite the advantages of the dividing wall column and despite much research and study, the processing industry has long felt reluctant to use dividing wall columns in commercial processes. This widespread reluctance has been attributed to various concerns, including control problems, operational problems, complexity, simulation difficulties, and lack of design experience. See, for example, the articles by C. Triantafyllou and R. Smith in Trans IChemE, Vol. 70, Part A, March 1992, 118-132; F. Lestak and C. Collins in Chem. Engg., July 1997, 72-76; and G. Duennebier and C. Pantelides in Ind. Eng. Chem. Res. 1999, 38, 162-176. The article by Lestak and Collins sets forth some general guidelines and considerations when substituting a dividing wall column for conventional columns. Nevertheless, the literature documents relatively few practical uses of dividing wall columns in commercial plants. See the article by H. Rudd in The Chemical Engineer, Distillation Supplement, Aug. 27, 1992, s14-s15 and the article in European Chemical News, Oct. 2-8, 1995, 26.
Prior art alkylaromatic processes, in particular, do not use dividing wall distillation columns. Nor do they use fully thermally coupled distillation columns, which as explained in the above-mentioned article by C. Triantafyllou and R. Smith, are thermodynamically equivalent to dividing wall columns when there is no heat transfer across the dividing wall. In particular, a dividing wall distillation column has not been used for producing the heartcut from the kerosene boiling range fraction. This is not only for the reasons given above but also for three additional reasons. First, the focus of prior research studies has been on separating relatively unchanging mixtures of only a few (e.g., 3 to 5) components, whereas the kerosene boiling range fraction is a seemingly ever-changing mixture of hundreds or thousands of hydrocarbons. Second, the research studies produce dividing wall distillation product streams containing usually only one component, whereas the heartcut, stripper overhead stream, and rerun bottom stream contain many hydrocarbon components. Third, product specifications for LAB require that the heartcut composition be controlled relatively tightly, since the detergent properties of the linear alkylbenzene sulfonates (LABS) depend in large part on the particular paraffin isomers in the heartcut. Thus, alkylaromatic processes are characterized by both an ever-changing composition of a complex kerosene mixture and a relatively tight specification on a complex mixture of paraffins in the heartcut. This combination compounds the problems, difficulties, and complexity of using a dividing wall distillation column or two fully thermally coupled distillation columns.
This invention is a process for the production of alkylaromatic hydrocarbons by alkylating feed aromatic hydrocarbons with linear olefinic hydrocarbons, where the linear olefinic hydrocarbons are produced by dehydrogenating linear paraffinic hydrocarbons, where the linear paraffinic hydrocarbons are extracted from a heartcut, and where the heartcut is fractionated from a kerosene fraction in either a dividing wall fractionation column or in two fully thermally coupled fractionation columns, where the two fully thermally coupled fractionation columns are a prefractionator and a main column. It has now been recognized that use of two fully thermally coupled fractionation columns or of a dividing wall fractionation column produces the heartcut in a manner that is stable and controllable for commercial LAB production, despite the complexity of the mixture of hydrocarbons in commercial kerosene fractions and despite the fluctuations in the compositions of those fractions. In addition, use of two fully thermally coupled fractionation columns or the dividing wall fractionation column reduces significantly the cost of utilities in producing the heartcut. In a preferred embodiment of this invention, the two fully thermally coupled fractionation columns or the dividing wall fractionation column is integrated with the paraffin recycle fractionation column and/or the LAB product fractionation column, thereby further decreasing the cost of utilities for producing LAB. As between a single dividing wall fractionation column on the one hand and two fully thermally coupled fractionation columns on the other hand, the former is preferred when the cost of a single fractionation vessel represents a significant savings over that of two fractionation vessels.
Accordingly, in a broad embodiment, this invention is a process for the production of alkylaromatics. A feed stream comprising low-boiling hydrocarbons, heartcut hydrocarbons, and high-boiling hydrocarbons passes into a first lateral section of an intermediate portion of a fractionation column at fractionation conditions. The entering compounds are separated to provide an overhead stream comprising the low-boiling hydrocarbons, a sidedraw stream comprising the heartcut hydrocarbons, and a bottom stream comprising the high-boiling hydrocarbons. The first lateral section is separated from a second lateral section of the fractionation column by a vertically oriented baffle extending upward from a lower portion of the fractionation column to an upper portion of the fractionation column. The overhead stream is at least partially condensed to form a condensed stream comprising the low-boiling hydrocarbons, and a portion of the condensed stream is refluxed to the fractionation column. The low-boiling hydrocarbons are recovered from the overhead stream. Heat is introduced to the lower portion of the fractionation column, and a bottom stream comprising the high-boiling hydrocarbons is withdrawn from the lower portion of the fractionation column. The high-boiling hydrocarbons are recovered from the bottom stream. A sidedraw stream comprising the heartcut hydrocarbons comprising paraffinic hydrocarbons is withdrawn from the second lateral section of the fractionation column. At least a portion of the sidedraw stream passes to a dehydrogenation zone to dehydrogenate the paraffinic hydrocarbons to monoolefinic hydrocarbons. A dehydrogenation zone effluent stream comprising the monoolefinic hydrocarbons is recovered from the dehydrogenation zone. At least a portion of the dehydrogenation zone effluent stream and an aromatic stream comprising a feedstock aromatic compound pass to an alkylation zone. The alkylation zone is operated at alkylation conditions to alkylate the feedstock aromatic compound with the monoolefinic hydrocarbons to produce alkylaromatic hydrocarbons. An alkylation effluent stream comprising the alkylaromatic hydrocarbons is recovered from the alkylation zone.
In another broad embodiment, this invention is a process for the production of alkylaromatics. A feed stream comprising low-boiling hydrocarbons, heartcut hydrocarbons, and high-boiling hydrocarbons passes into a prefractionator fractionation column which separates the entering hydrocarbons to provide a prefractionator overhead vapor stream comprising the low-boiling hydrocarbons and the heartcut hydrocarbons and a prefractionator bottom liquid stream comprising the high-boiling hydrocarbons and the heartcut hydrocarbons. At least a portion of the prefractionator overhead vapor stream passes to a main fractionation column, which is fully thermally coupled to the prefractionator fractionation column. At least a portion of the prefractionator bottom liquid stream passes to the main fractionation column. Hydrocarbons are separated in the main fractionation column. The following five streams are recovered from the main fractionation column: a main column overhead stream comprising the low-boiling hydrocarbons, a main column bottom stream comprising the high-boiling hydrocarbons, a main column product sidedraw stream comprising the heartcut hydrocarbons, a main column upper sidedraw stream comprising the low-boiling hydrocarbons and the heartcut hydrocarbons, and a main column lower sidedraw stream comprising the heartcut hydrocarbons and the high-boiling hydrocarbons. At least a portion of the main column upper sidedraw stream passes to the prefractionator fractionation column. At least a portion of the main column lower sidedraw stream passes to the prefractionator fractionation column. The main column overhead stream is at least partially condensed to form a condensed stream comprising the low-boiling hydrocarbons. A portion of the condensed stream is refluxed to the main fractionation column. Low-boiling hydrocarbons are recovered from the main column overhead stream. Heat is introduced to a lower portion of the main fractionation column, the high-boiling hydrocarbons are recovered from the main column bottom stream. At least a portion of the main column product sidedraw stream passes to a dehydrogenation zone, where the paraffinic hydrocarbons are dehydrogenated to monoolefinic hydrocarbons. A dehydrogenation zone effluent stream comprising the monoolefinic hydrocarbons is recovered from the dehydrogenation zone. At least a portion of the dehydrogenation zone effluent stream and an aromatic stream comprising a feed aromatic hydrocarbon pass to an alkylation zone operated at alkylation conditions to alkylate the feed aromatic hydrocarbon with the monoolefinic hydrocarbons to produce an alkylation effluent stream comprising alkylaromatic hydrocarbons. The alkylaromatic hydrocarbons are recovered from the alkylation effluent stream.
Other embodiments of the invention are set forth in the detailed description of the invention.